[TaskScheduler] Require constant initialization for g_heartbeat_for_testing

Somehow r562248 caused a regression whereby it should have been strictly
more performant, the only difference is the potential static
initializer (which if incorrectly initialized could result in it being
non-zero? and somehow cause the timer to be fast?).
Let's try to suppress it.

R=fdoray@chromium.org

Bug: 848255
Change-Id: I21111ddd4c069b9171badc58fc4a1914bb068641
Reviewed-on: https://chromium-review.googlesource.com/1090794Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578420}

base/task_scheduler/service_thread.cc

@@ -13,13 +13,19 @@
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/task_scheduler/task_tracker.h"
 #include "base/task_scheduler/task_traits.h"
+#include "build/build_config.h"
 
 namespace base {
 namespace internal {
 
 namespace {
 
-TimeDelta g_heartbeat_for_testing = TimeDelta();
+// TODO(gab): Verify whether this attribute addresses https://crbug.com/848255
+// and consider a way to expose it more widely.
+#if defined(__clang__) && !defined(OS_NACL)
+[[clang::require_constant_initialization]]
+#endif
+    TimeDelta g_heartbeat_for_testing = TimeDelta();
 
 }  // namespace